The art of inkjet printing technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines successfully employ inkjet technology for producing hard copy printed output. The basics of the technology have been disclosed in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Inkjet devices have also been described by W. J. Lloyd and H. T. Taub in Output Hardcopy Devices (R. C. Durbeck and S. Sherr, ed., Academic Press, San Diego, 1988, chapter 13).
A thermal inkjet printer for inkjet printing typically includes one or more translationally reciprocating print cartridges in which small drops of ink, are ejected by thermal energy from a drop generator, towards a medium upon which it is desired to place alphanumeric characters, graphics, or images. Such cartridges typically include a print head having an orifice member or plate that has a plurality of small nozzles through which the ink drops are ejected. Beneath the nozzles are ink firing chambers, which are enclosures in which ink resides prior to ejection through a nozzle. Ink is supplied to the ink firing chambers through ink channels that are in fluid communication with an ink reservoir, which may be contained in a reservoir portion of the print cartridge or in a separate ink container spaced apart from the print head.
Ink drop ejection through a nozzle employed in a thermal inkjet printer is accomplished by quickly heating the volume of ink residing within the ink firing chamber with a selectively energizing electrical pulse to a heater resistor ink ejector positioned in the ink firing chamber. At the commencement of the heat energy output from the heater resistor, an ink vapor bubble nucleates at sites on the surface of the heater resistor or its protective layers. The rapid expansion of the ink vapor bubble forces the liquid ink through the nozzle. Once the electrical pulse ends and an ink drop is ejected, the ink firing chamber refills with ink from the ink channel and ink reservoir.
Thermal inkjet ink can be corrosive. Prolonged exposure of electrical interconnections of an ink cartridge to the ink, will frequently result in a degradation and failure of the print head because the transistors that fire the heater resistors are effectively cut off from their source of power or from their control signals. In some print head designs, the transistors that fire the heater resistors are addressed (controlled) from a single electrical connector. If this one connector is electrically disabled because of chemical attack from the ink and its constituents, a large part (or all) of an ink cartridge can fail, adversely affecting print quality.
The heater resistors of a conventional inkjet print head comprise a thin film resistive material deposited on an oxide layer of a semiconductor substrate. Electrical conductors are patterned over the oxide layer and provide an electrical path to and from each thin film heater resistor. Since the number of electrical conductors can become large when a large number of heater resistors are employed in a high density (high DPI--dots per inch) print head, various multiplexing techniques have been introduced to reduce the number of conductors needed to connect the heater resistors to circuitry disposed in the printer. See, for example, U.S. Pat. No. 5,541,629 "Print head with Reduced Interconnections to a Printer" and despite its good conductivity, imparts an undesirable amount of resistance in the path of the heater resistor.
Individual transistors are typically addressed using combinations of electrical signals applied to the drain, source and gate terminals. These combinations of signals can effectively control when individual transistors will be in their "on" state, thereby allowing a droplet of ink to be ejected onto the print medium. Multiplexing the function of the various lines through the semiconductors allows a large number of individual transistors to be addressed using a relatively small number of address line conductors.
Multiplexing techniques have helped reduce the total number of conductors necessary to energize the heater resistors. Notwithstanding the improvements in addressing, more improvement is needed, however, to reliably address each transistor to avoid catastrophic failure of a print head caused by a single fault on an address bus. In addition, there is a need to provide printheads that have a flexibility to accept different input signal configurations.